pH changes in peralkaline late-magmatic fluids

The 1.15-Ga-old Ilimaussaq intrusive complex in South Greenland shows an extensive fractionation trend from alkaline augite syenite to various varieties of strongly peralkaline, agpaitic nepheline syenites. The peralkaline nepheline-bearing syenites crystallized between ca. 900 and 450 °C at 1 kbar and they are cut by late-magmatic hydrothermal veins with nepheline-absent assemblages of albite + aegirine + analcime - sodalite - Na-Be-silicates (tugtupite, chkalovite, sørensenite) - ussingite (NaAlSi₃O₈*NaOH). Based on fluid inclusions and phase equilibria, these veins crystallized between 300 and 500 °C at 1 kbar. Textures indicate that the hydrothermal veins at least partly replaced earlier Ilimaussaq rocks. The occurrence of ussingite and tugtupite suggests that the late-magmatic fluids had strongly basic pH values. Speciation calculations show that the pH in fluids of the system Na-Al-Si-O-H-Cl mainly depends on the Na/Cl ratio and, to a lesser degree, on salinity and temperature. If the Na/Cl ratio is greater than 1, pH (at 400 °C and 1 kbar, where neutrality is about at pH 5) lies between 7 and 12. Because Na/Cl tends to decrease in the final stages of magmatism and during crystallization of the vein assemblage, pH of late-magmatic fluids generally should become more acidic, and only two processes can increase Na/Cl and, thus, pH: dissolution of a Cl-poor or Cl-free Na silicate or unmixing of an HCl-enriched vapour phase. Field observations and microtextures suggest that replacement reactions are responsible for the change to basic pH at least in some alteration assemblages. While replacement of 1 mol nepheline by 1 mol analcime would not alter the pH, the volume-conserving reaction 1.85  Ne + 2.3  H₂ O + 0.19  H₄ SiO₄ = 1.02  Anl + 0.83   Na⁺ + 0.83  Al(OH)₄^-1.85{\rm Ne} + 2.3{\rm H}_{\rm 2} {\rm O} + 0.19{\rm H}_{\rm 4} {\rm SiO}_4 = 1.02{\rm Anl} + 0.83\,{\rm Na}^ + + 0.83{\rm Al(OH)}_4^ - can be used to model the replacement process quantitatively, provided it occurred in a more or less closed system. Progress of this reaction leads to successively increasing pH of the fluid during fluid-rock interaction and stabilizes minerals such as ussingite and tugtupite. Transferring the two processes to a larger scale, it is proposed that the extreme 'hyper-agpaitic' assemblages at Ilimaussaq or at the Kola peninsula, which include copious amounts of very basic, water-soluble minerals such as trona, villiaumite or thermonatrite, are formed either in this way by autometasomatic reactions of late-magmatic fluids or melts (or supercritical fluid-melt-mixtures) with earlier crystallized rocks of the same plutonic complex or by large-scale vapour unmixing in the very final stages of magmatism.